One of the features of this invention is to provide an improvement in the preparation of large pore volume xerogels or xerocogels which are useful as polyolefin catalyst supports, some of which possess improved thermal stability. Chromium(II), chromium(III) or chromium(VI) compounds based on these supports are catalysts that are capable of producing polyethylene resins of melt index greater than about eight.
Another feature of this invention is to prepare an improved large pore volume zirconia-silica catalyst support by reacting a particular zirconium compound with a particular silicate to produce a hydrocogel, aging the hydrocogel, washing with water and then aqueous ammonium nitrate and again with water, removing water from the resulting washed hydrocogel to produce a xerocogel either by azeotropic distillation or by leaching with a water miscible solvent, after which residual water is removed by chemically reacting this water with a ketal of the above formula RC(OR).sub.2 R, and then calcining the resulting xerocogel.
Another feature of the invention is to provide a catalyst support prepared by the above method.
A further feature of the invention is a method of polymerizing olefins comprising contacting the olefins with a catalyst comprising the above support and a chromium compound associated with it under polymerizing conditions.
The following prior art is believed to be the most pertinent:
Aboutboul et al U.S. Pat. No. 3,978,002 discloses a chromium(VI)oxide olefin polymerization catalyst comprising a silica xerogel support obtained by dehydrating the corresponding hydrogel by extraction with acetone. A polymerization process using such a catalyst support for the production of ethylene polymers produces polyethylene resins having relatively low molecular weights and, therefore, high melt index in the range 3.2-4.2 (obtained without hydrogen modification).
Bachl et al U.S. Pat. No. 4,042,770 relates to a process for the manufacture of olefin polymers with the aid of a specially prepared silica-chromium(VI)oxide catalyst. The specially prepared silica hydrogel is dehydrated by extraction with an organic liquid selected from C.sub.1 -C.sub.4 alkanols and/or C.sub.3 -C.sub.5 alkanones (ketones) until the organic liquid absorbed no more water. The dehydrated gel is then freed of organic liquid by drying at 120.degree. C. to give the final xerogel. Doping with chromium(VI)oxide and air activation resulted in catalysts which produced ethylene polymers with very low melt index.
Short et al U.S. Pat. No. 4,081,407 describes a method for treating a hydrocogel consisting of coprecipitated silica-titania, 2.5 wt.% titanium (U.S. Pat. Nos. 3,950,316 and 3,862,104), with a saturated aliphatic alcohol selected from the group consisting of 1-pentanol, 3-methyl-1-butanol, 4-methyl-2-pentanol, 2,2-dimethyl-1-propanol, and 1-hexanol to remove water azeotropically or by extraction. The dried product retains a porous structure and is particularly suitable upon impregnation with a polymerization catalyst component, such as chromium(VI)oxide, for use in promoting the production of ethylene polymers of melt index in excess of about 6.6 (highest value disclosed is 7.5). These inventors discovered that the specific solvents disclosed removes water from the hydrocogels and affects the melt index of the polymer produced in polymerization reaction using catalysts supported on these strata.
Hwang et al U.S. Pat. No. 4,128,500 discloses chromium(III)acetylacetonate type catalysts. Dombro et al patent application Ser. No. 44,004, filed May 31, 1979, describes novel methods of preparing zirconia-silica xerocogels. Both are assigned to the assignee hereof.